Obama described the move a clear violation of Ukrainian sovereignty and territorial integrity -- a breach of international law, including the United National Charter and Russia's 1997 military basing agreement with Ukraine.

The two men spoke for 90 minutes Saturday afternoon after a unanimous vote to send troops to Crimea, a Russian-majority region in Ukraine. Putin made a written statement outlining the reasons such an audacious move was necessary.

"The United States condemns Russia's military intervention into Ukrainian territory," according to a statement released from the White House. "The United States calls on Russia to de-escalate tensions by withdrawing its forces back to bases in Crimea and to refrain from any interference elsewhere in Ukraine."

Obama told Putin that Russia's continued violation of Ukraine's sovereignty "would negatively impact" Russia's standing in the international community. The U.S. also announced it was suspending its participation in preparatory meetings for the G-8 to be held in Russia later this year.

"Russia's continued violation of international law will lead to greater political and economic isolation," according to the statement.

The United States has consistently said it recognizes Russia's historic and cultural ties to Ukraine and the need to protect the rights of Russian and minority populations within Ukraine," according to the White House. And, it said, the Ukrainian government is commited to protect the rights of all Ukrainians.

Obama told Putin that, if Russia has concerns about the treatment of ethnic Russian and minority populations in Ukraine, the appropriate way to address them is peacefully through direct engagement with the government of Ukraine and through the dispatch of international observers through the U.N. Security Council or the Organization for Security and Cooperation in Europe. As a member of both organizations, Russia would be able to participate.

Obama urged an immediate effort to initiate a dialogue between Russia and the Ukrainian government.

In the coming days, the U.S. will consult with allies on the UN Security Council, the North Atlantic Council, the Organization for Security and Cooperation in Europe, and with the signatories of the Budapest Memorandum while continuing to provide support for the Ukrainian government, including urgent technical and financial assistance.

"We will continue consulting closely with allies and partners, the Ukrainian government and the International Monetary Fund, to provide the new government with significant assistance to secure financial stability, to support needed reforms, to allow Ukraine to conduct successful elections, and to support Ukraine as it pursues a democratic future," the White House said.

Obama also spoke to French President Francois Hollande and Canadian Prime Minister Stephen Harper. The leaders expressed "grave concern" over Russia's action in Ukraine.

Obama's national security team met earlier Saturday to discuss the ongoing crisis. Obama did not attend but was briefed by National Security Adviser Susan Rice.

Outside the White House, a small group of Ukrainian protesters called on the U.S. and the European Union to do more to combat Russian aggression in Crimea.

Ilona Doerfler, a young woman from Kiev who carried the megaphone, urged economic sanctions and a naval blockade.

“NATO ships in the Black Sea should organize a non-invasive blockade to stop Russian ships exiting,” she said. Asked about the dangers of escalation, she replied: “It's already escalating. It is not about the Ukraine, it's about the integrity of promises to respect territorial sovereignty.”